Its not like it was a seperate piece of software that got Doom back online, which could then be repurposed and pluged into any game. The fact that Doom works on BBA certainly means that a programmer could get a different game online. However Doom's code could only be used as a reference. Its like a hotdog bun versus a hamburger bun. Sure they both hold meats, but they are shaped differently. (best analogy I can think of).

You can't tunnel the game over XLink because it doesn't have a LAN function that scans the local network for games, or a local master list view. The game works via direct IP input only, and all users have to manually input everyone's IP addresses.